The present invention relates to a rotary type electrostatic spray painting device.
One known electrostatic spray painting device which has been used for painting, for example, bodies of motor cars, is of the rotary type and comprises a rotary shaft supported by ball bearings or roller bearings arranged within the housing of the painting device, and a cup shaped spray head fixed onto the front end of the rotary shaft. In this painting device, a negative high voltage is applied to the spray head, and paint is fed onto the inner circumferential wall of the spray head. Thus, fine paint particles charged with electrons are sprayed from the spray head and are attracted by an electrostatic force onto the surface of the body of a motor car, which is grounded. As a result of this, the surface of the body of a motor car is painted. In such a rotary type electrostatic spray painting device, about 90 percent of the paint sprayed from the spray head, can be efficiently used for painting the surface to be painted. Thus the amount of the paint which is wasted is small and, as a result, rotary type electrostatic spray painting devices are used in various industries.
In order to form a beautifully finished painted surface, it is necessary to prevent the spray paint from forming air bubbles. It is also necessary to reduce the size of the particles of paint as much as possible. However, in a conventional rotary type electrostatic spray painting device, since spray paint contains air bubbles therein, a large number of air bubbles are present within the paint layer formed on the surface to be painted and, as a result, it is difficult to form a beautifully finished surface.
In order to prevent the spray paint from developing air bubbles, another rotary type of electrostatic spray painting device has been proposed, in which a plurality of grooves is formed on the inner wall of the tip of the spray head. In this rotary type electrostatic spray painting device, since paint is spouted from the grooves of the spray head in the form of a filament and then divided into fine particles, it is possible to prevent the resulting fine particles from containing air bubbles. However, in this known rotary type electrostatic spray painting device, the filaments and particles of paint are relatively large size, and it is therefore difficult to form a beautifully finished surface.
As mentioned above, in order to form a beautiful finished surface when the surface is painted by using sprayed paint, it is necessary to reduce the size of the particles of paint as much as possible. In the case wherein the paint is divided into fine particles by using the centrifugal force caused by the rotation of a spray head in a rotary type spray painting device, the strength of the centrifugal force, which is related to the rotating speed of the spray head, has a great influence on the size of the particles of paint. In other words, the higher the rotating speed of the spray head, the smaller the size of the particles of paint becomes. Consequently, in order to form a beautifully finished surface by using a rotary type electrostatic spray painting device, it is desirable to increase the rotating speed of the spray head as much as possible. In conventional rotary type electrostatic spray painting devices, ball bearings or roller are used for supporting the rotary shaft of the electrostatic spary painting device and, in addition, a lubricant, such as grease, is confined within the bearings. However, when such bearings, which are lubricated by grease, are rotated at a high speed, the bearings instantaneously deteriorate. Therefore, in conventional rotary type electrostatic spray painting devices which use bearings which are lubricated by grease, the maximum rotating speed of the rotary shaft, and consequently of the spray head, is at most 20,000 r.p.m. However, in known arrangements where the rotating speed of the spray head is about 20,000 r.p.m., the size of the particles of paint is relatively large and, thus, it is difficult to form a beautifully finished painted surface.
The painting process for the bodies of motor cars comprises a primary spraying step, an undercoating step, and a finish painting step. However, since it is difficult to form a beautifully finished surface by using a conventional rotary type electrostatic spray painting device, such a device is used for carrying out the undercoating step, but not the finish painting step.
One known system for lubricating ball and roller bearings injects lubricating oil of a low viscosity into the region between the inner race and the outer race of the bearing. In this manner the friction between the balls or rollers and such races is greatly reduced, and the heat caused by the friction is absorbed by the lubricating oil. In situations where the above-mentioned jet lubricating system is applied to a rotary type electrostatic spray painting device, it is possible to increase the rotating speed of the rotary shaft of the electrostatic spray painting device over the speeds attainable when grease lubricated bearings are used. However, since the jet lubricating system requires a complicated and bulky lubricating oil feed arrangement, it is particularly difficult to apply such a jet lubricating system to a rotary type electrostatic spray painting device. In addition, if the lubricating oil mixes with the paint, the appearance of the painted surface is damaged. It is practically impossible to prevent the lubricating oil from leaking into the paint and, thus, it is inadvisable to apply the jet lubricating system to a rotary type electrostatic spray painting device.
A known electrostatic spray painting device which is capable of reducing the size of the particles of paint to a great extent while preventing the paint particles from containing air bubbles therein, divides the paint into fine particles by injecting a stream of air. In this air injection type electrostatic spray painting device, since the size of the particles of sprayed paint can be reduced to a great extent, as mentioned above, it is possible to form a beautifully finished surface. Consequently, the air injection type electrostatic spray painting device has been used for carrying out the finish painting step for the bodies of motor cars. It is a problem with the known air injection type electrostatic spray painting devices that the sprayed paint impinges upon the surface to be painted together with the stream of the injection air and therefore substantial portion of the sprayed paint escapes into the atmosphere with the stream of the injection air. The amount of the paint which adheres to the surface to be painted is only about 40 percent of the paint sprayed from the electrostatic spray painting device. Consequently, air injection type electrostatic spray painting devices suffer from the problem in that the consumption of the paint is inevitably increased. A further problem occurs in that the paint which escapes with the stream of the injection air, causes air pollution within factories.
It is therefore an object of the present invention to provide a rotary type electrostatic spray painting device which reduces the size of the particles of paint to be sprayed; reduces the quantity of paint used; and prevents paint particles from containing air bubbles.